There are a considerable number of vibration testing systems ("shakers") that are well-known in the prior art. These shakers are used to mechanically shake an item for the purpose of diagnostically testing responses to certain driving forces. The item is physically attached to a moving portion of the shaker and when the shaker is activated, the item is subjected to a variety of test conditions. The moving portion of the shaker is typically driven by a force which may be continuous, cyclical or impulsed. One class of these shakers employs the use of an electromagnetic field between field and armature windings. Various driving signals are impressed across the armature winding to control the movement of the shaker.
Vibration testing systems are employed to test the effects of vibration on various types of equipment and component parts. In vibration test systems which employ direct current magnetic fields to produce the required vibration energy high magnitude flux densities are produced. These high magnetic flux densities are also present in the test environment area of the vibration testing system which adversely affects the equipment or part being tested. To reduce the adverse effects of this high magnetic flux density in the test environment area, such vibration testing systems are usually provided with a suitable degaussing coil. The degaussing coil is positioned between the vibration testing system structure and the table thereof upon which the specimen to be tested is mounted. The degaussing coil operates by generating a magnetic field in opposition to the field produced by the vibration testing system which tends to cancel the undesired field.
In many situations in spite of the action of the degaussing coil, the level of the stray magnetic field is still high enough to adversely affect the equipment or part being tested. For example, the magnetic field produced by the magnetic structure (body) of the vibration testing system is due to saturation of the body's field. This saturated field creates a leakage flux that is not proportional to the field supply voltage. The degaussing coil (not saturated), however, produces a field which is proportional to the supply current. Accordingly, as the supply voltage varies with line voltage, and the coil resistance varies with coil temperature, there is a residual stray field which can be detrimental to a desirable test environment.